Solid lubricants typically comprise a relatively thin film of at least one solid which is applied to one or more moving, interfacing surfaces. Although lubricants in general function to reduce friction and/or wear between such surfaces, solid film lubricants have been used in a wide variety of applications requiring additional performance characteristics. U.S. Pat. Nos. 3,014,865; 3,674,690; 3,778,308; 3,862,860; 4,202,780; 4,338,376; 4,474,669; 4,828,729; and 4,892,669 are generally representative of various applications for lubricants.
One application for solid film lubricants is to provide lubrication between load-bearing surfaces. such as in gears, bearings, and between sliding metal plates. In each of these types of applications, a significant load may be communicated through the solid film lubricant. Therefore, it is desirable not only for the solid film lubricant to provide for a combination of friction and wear reduction between the interfacing surfaces, but also for the lubricant. to adequately perform these functions under load conditions for an extended period of time. That is, the wear life of the lubricant is also an important factor in evaluating the lubricant's overall performance.
There are generally two basic types of solid lubricants. Unbonded solid lubricants (e.g., in the form of a powder), are typically directly applied to a surface to be lubricated and adhere thereto by some degree of mechanical or molecular action (i.e., the lubricant is not physically or chemically bonded to the surface being lubricated). Consequently, the properties of the solids themselves will generally define the performance characteristics for the given application. However, since there is no physical or chemical bonding of the solid lubricant to the surface, the potential exists, particularly in load-bearing applications, that the unbonded solid lubricant will not remain in position to provide the desired performance over an extended period of time.
An alternative solid lubricant to the above is a bonded solid lubricant. These lubricants are physically or chemically attached to the desired surface by an adhesive or binder. Generally, the solid lubricant is mixed with the particular adhesive/binder and applied to one or more of the surfaces to provide a film of a desired thickness. Depending upon the application, proper selection of the adhesive/binder may be important.
One consideration in adhesive/binder selection is the manner in which the adhesive/binder is cured. For instance, some adhesives/binders require thermal curing at relatively high temperatures for significant periods of time. Consequently, this typically requires that the part being lubricated be positioned in a curing oven after application of the lubricant thereto. Although this may be acceptable in certain applications, it may not be in the case where relatively large components are involved and/or when removal of the part to be lubricated is cumbersome. As a result of these types of disadvantages, air curable adhesives/binders have been used in certain applications. In this case, lubrication may often take place in situ and after an appropriate cure time at room temperature the solid film lubricant will be ready for use.
Another consideration in adhesive/binder selection is the manner in which the cured solid film lubricant may be removed. For instance, in many cases the initial application of the solid film lubricant is not satisfactory such that the lubricant must be removed and reapplied. Many adhesives/binders used with solid lubricants are non-soluble such that they must be physically ground or mechanically removed from the lubricated surface(s) (e.g., when an epoxy is used). As can be appreciated, this greatly affects lubrication procedures and expenses. Moreover, in the event that a thermally curable adhesive/binder is being utilized, this further increases lubrication costs since once again the lubricated part is typically placed in an oven for an extended period of time (e.g., 56 hours at 300.degree. C. in the case of a solid lubricant which includes molybdenum disulfide, antimony trioxide and a polyimide binder).
The binder not only impacts application and removal requirements, but may also affect the performance characteristics of the solid film lubricant. For instance, in the case of load-bearing applications it has been generally accepted theory that the binder be selected and the lubricant applied such that the cured solid lubricant film will be as thin and hard as possible. Consequently, the lubricant effectively communicates the load and does not itself carry any of the load. One problem associated with this particular theory is that the wear life of the lubricant is adversely affected and unacceptable in certain applications. More particularly, the surface of the cured solid film lubricant has a certain roughness and is defined by a series of asperities (i.e., tiny peaks) with cavities interspersed therebetween. These asperities often interlock with the cavities on the interfacing surface. Consequently, when there is relative motion between these surfaces, over time, if not initially, there is a tendency for the asperities to break off. This affects the performance of the lubricant and ultimately its own wear life.
Based upon the foregoing, it should be appreciated that binder selection is an important aspect relating to bonded solid film lubricants for particular applications. Not only does the binder effectively dictate the lubrication application/removal characteristics of the solid film lubricant, but it also impacts the performance characteristics of the lubricant as well.